Process and composition for the production of protective coatings



United States Patent U.S. Cl. 148-6.15 18 Claims ABSTRACT OF THE DISCLOSURE A coating composition and method whereby uniform and finely grained manganese phosphate coatings having a thickness of less than 3 millimicrons are applied to ferrous metal articles and such coated articles. The composition contains an oxidising agent and has a ratio of free P 0 to total P 0 which is greater than 0.3 and less than 0.6.

This invention relates to a process and composition for the production of protective coatings on ferrous surfaces, and more particularly it relates to a process and composition for producing thin, fine-grained, firmly adhering manganese phosphate coatings on surfaces such as iron and steel.

It has been known that because of their high mechanical stability, coatings for ferrous surfaces which are predominantly manganese phosphate are useful in a variety of applications. For example, such manganese phosphate coatings are effective in reducing the friction of metal surfaces sliding on one another and also facilitate cold forming of metal. When utilizing manganese phosphate solutions in treating ferrous surfaces, however, the coatings obtained are normally in the form of relatively thick, coarse crystalline layers. In some instances, these thick coarse grained coatings are disadvantageous, as for example, in the case of mechanical parts which are required to have close dimensional tolerances, since the dimensions of such parts may be changed appreciably by the heavy coating. It is, therefore, desirable to produce maganese phosphate coatings which are thin and finely crystalline.

In an effort to accomplish this result, it has been proposed to add condensed phosphates to the maganese phosphate solutions, thereby obtaining a refinement of the phosphate layer produced. Generally, however, manganese phosphate coating solutions are used at relatively high temperatures, at which temperatures the added condensed phosphates hydrolyze to a considerable degree. Accordingly, the operation of such coating baths has been found to be diificult in many instances.

It has also been known that manganese phosphate coating solutions which contain oxidizing agents generally produce thinner coatings than are obtained from such coating solutions which are free of oxidizing agents. In this regard, it has been proposed that there be used manganese phosphate solutions containing nitrate in which the amount of nitrate, calculated at N0 exceeds the amount of phosphate, calculated as P0,. In such solutions the weight ratio of N0 to P0 is given as preferably being in the range of about 1.5 to 45:1. With such solutions, it is alleged that extremely dense, fine-grained phosphate coatings with a thickness of about 5 millimicrons are obtained.

It has further been proposed to produce zinc phosphate or manganese phosphate coatings on ferrous surfaces using zinc phosphate or manganese phosphate solutions which contain at least 1% of nitrate and which are main- 3,450,578 Patented June 17, 1969 tained practically free of ferro-iron and at a low bath density. In these solutions, it is indicated that a ratio of free acid to total acid of l to at least 5.5 is maintained. When using either of these methods, however, it is found that in many instances, the reduction in the weight of the manganese phosphate layer obtained is not sufficient to prevent undesirable variations in the dimensions of ferrous parts treated.

It is, therefore, an object of the present invention to provide a novel composition from which thin, fine-grained firmly adhering manganese phosphate coatings may be produced on ferrous surfaces.

A further object of the present invention is to provide an improved method for treating ferrous surfaces, whereby a thin, fine-grained firmly adhering manganese phosphate coating may be obtained on the surfaces treated.

These and other objects will become apparent to those skilled in the art from the description of the invention which follows.

Pursuant to the above objects, the present invention includes an aqueous acid solution comprising manganese phosphate and an oxidizing agent and having a ratio of free P 0 to total P 0 greater than 0.30 and less than 0.60. Desirably, the ratio of free P 0 to total P 0 in the solutions in maintained between about 0.35 and about 0.55 and is preferably within the range of about 0.40 to about 0.50.

More specifically, the composition of the present invention comprises an aqueous acidic manganese phospate solution wherein the manganese is typically present in an amount within the range of about 3 to about 20 grams per liter and the P 0 is typically present in an amount within the range of about 4 to about 25 grams per liter. Typically, such acid manganese phosphate solutions are formed from manganese phosphate and/or manganese acid phosphate and/ or phosphoric acid. Solutions of this general type are conventional and are known to those in the art.

In addition to the acid manganese phosphate, the solution also contains an oxidizing agent. These oxidizing agents generally act as accelerators to reduce the time required to obtain a uniform, substantially continuous coating. These materials are exemplified by the nitrates, nitrites, halides, such as the bromides, chlorates, bromates, sulfites, perchlorates, iodates, peroxides, periodates, permanganates, organic nitro compounds such as m-nitrobenzene sulfonate, nitroguanidine, nitromethan, nitroethane, l-nitropropane, nitrobenzene, o-nitrobenzaldehyde, p-nitrophenol, p-nitroaniline, p-nitrochlorobenzene, picric acid, and the like. Of these, excellent results have been obtained when using a nitrate as the oxidizing agent and for this reason materials of this type are preferred. Typically, when the oxidizing agent used in a nitrate, it is present in the aqueous acid manganese phosphate solution in an amount within the range of about 2 to about 50 grams per liter of N0 Additionally, in some instances, it has been found to be desirable if the aqueous acid manganese phosphate composition of the present invention also contains nickel ions. Accordingly, in the most preferred embodiment of the present invention, the manganese phosphate solutions will also contain nickel ions, desirably in an amount 'Within the range of about 0.2 to about 1 gram per liter.

Inasmuch as the compositions of the present invention are desirably in the form of an aqueous solution, the above and other adjuvants which may 'be included in the composition are preferably in a form which is readily dispersible in the aqueous solution. Accordingly, it will be appreciated that many water dispersible forms of these compounds may be used, provided they have no significantly adverse effect on the coating composition,

the metal substrate or the coating subsequently produced or applied.

In maintaining the desired ratio of free P to total P 0 in the present composition, the amount of free P 0 may be determined in the following manner:

A milliliter solution of the bath is titrated with 0.1 normal sodium hydroxide until the end point of the first stage of dissociation of H PO is reached. To indicate this end point, indicators such as dimethyl yellow, or methyl orange may be used. In this determination, l milliliter of the 0.1 normal sodium hydroxide used has been found to correspond to 0.71 gram per liter of free P 0 Additionally, the value for total P 0 can be determined using various known methods for phosphate analysis. For example, the total P 0 may be determined by the precipitation of the phosphate with ammonium molybdate followed by weighing of the precipitate.

It will be appreciated that the maintaining of the various desired ratios of constituents of the present composition can be effected by the choice of the components which are added to the composition. In many instances, it has been found to be desirable to adjust the ratio of free P 0 to total P 0 by the addition of nitric acid to the phosphatizing solution. It is, of course, possible to adjust this ratio also by the addition of other materials containing non-interfering anions, such as $0., or C1 or noninterfering cations, such as the alkali metals, e.g., sodium, potassium, and the like. It has been found, however, that when using nitric acid to control the ratio of free P 0 to total P 0 in the solution, there is a relationship established involving the free acid in the solution, which relationship is expressed by the following empirical formula:

Free Acid divided by R=percent P0 divided by 0.48 where the free acid is the milliliter of a 0.1 normal sodium hydroxide (points) determined by titrating a two milliliter sample with 0.1 normal sodium hydroxide to the end point of an indicator of methyl orange and xylene cyanole in water and isopropyl alcohol, R is the ratio of free P 0 to total P 0 and the percent P0 is the percent of P0 available in the bath. Additionally, as used hereinafter, the total acidity of the solutions is the milliliter of 0.1 normal sodium hydroxide (points) determined by titrating a two milliliter sample of the solution with 0.1 normal sodium hydroxide to the phenolphthalien end point.

-In carrying out the process of the present invention, ferrous surfaces, such as iron and steel, are contacted with the subject aqueous acid manganese phosphate solution wherein water is the aqueous part of the solution. This contact between the ferrous surface to be coated and the manganese phosphate solution may be effected in various ways, as for example, by immersion, by spraying, by flooding, by roller coating and the like. Preferably, however, the articles to be coated are dipped into or immersed in the manganese phosphate solution. Desirably, the manganese phosphate solution is at an elevated temperature and preferably at a temperature close to its boiling point, temperatures within the range of about 90 to about 97 degrees centigrade being typical. The ferrous article to be coated is maintained in the acid manganese acid phosphate solution for a period of time sufficient to obtain the desired coating thereon. Contact times of about 0.5 to about minutes are typical with up to about 2 minutes generally being sufficient.

Generally, the surfaces to be treated with the composition of the present invention are cleaned prior to being brought in contact with the phosphatizing solution. Various cleaning techniques and cleaning compositions may be used, although in many instances, cleaning with organic solvents, such as trichloroethylene or perchloroethylene has been found to be advantageous. After cleaning of the surface has been completed, it has generally been found to be desirable to rinse the surface with water prior to phosphatizing. After the articles are removed from the acid manganese phosphate solution, they may be rinsed with water and, if desired, passivated in the known manner, using conventional passivating solutions, such as those containing chromium, e.g., chromic acid.

As has been noted hereinabove, the solutions of the present invention are particularly suitable for application on ferrous parts which must maintain a close dimensional tolerance and in which only small variations in dimension are permitted. Exemplary of parts with which the process of the present invention may be used are piston rings, and parts for refrigerator compressors, typewriters, and the like. Additionally, because of the fine grain and adhesive strength of the coatings produced by the present method, it has been found that these coatings greatly facilitate the sliding of metal surfaces on one another, i.e., the coatings reduce friction between sliding metal surfaces.

In order that those skilled in the art may better understand the present invention and the manner in which it may be practiced, the following specific examples are given. It will be appreciated, of course, that these examples are merely exemplary of the process and composition of the present invention and are not to be taken as a limitation of the invention. Unless otherwise indicated, in these examples the temperatures are give in degrees Centigrade and amounts of materials are given in grams per liter.

EXAMPLES 1-8 In these examples, sliding steel pistons used for the manufacture of refrigerator compressors were degreased with perchloroethylene vapors and rinsed in hot water for one minute at 90 degrees centigrade. Thereafter, the parts were immersed for two minutes at 95 degrees in an aqueous solution containing 6.4 grams per liter manganese, 9.7 grams per liter P 0 8.9 grams per liter N0 and 0.2 gram per liter Ni. Several runs were made wherein the ratio of free P 0 to total P 0 in the aqueous acid solutions was adjusted by the addition of concentrated nitric acid to the solution. After the treated parts were removed from the solutions they were rinsed and dried and the thicknesses of the manganese phosphate coatings obtained were measured. Using this procedure, the following results were obtained.

Amount of HNOz added to solution Ratio of tree P205 Coating thickness Example (grams/liter) to total P205 (millirnicrons) None 0.30 3. 0 0. 41 0. 35 1. 7 0.82 0. 46 1. 6 1. 23 0. 1. 2 1. 64 0 50 1. 3 2. 05 0. 1. 7 2. 46 0. 1 2. 0 2. 87 0.

1 Coating powdery and can be wiped off. 2 No coating formed.

EXAMPLES 9-13 In these examples, the procedure of Examples l-8 was repeated with the exception that the quantity of nickel ions in the manganese phosphate solution was varied and the ratio of free P 0 to total P 0 was maintained at 0.45 by the addition of nitric acid. Using this procedure, the following results were obtained.

Amount of Ni++ added to solution Coating thickness Example (grams/liter) (millimierons) None 2.

EXAMPLE 14 A manganese phosphate solution concentrate was formulated by combining 250 grams of a 50 percent solution of manganese nitrate, 127 grams of manganese acid phosphate, 62 grams of percent phosphoric acid, and 373 grams of water. An aqueous working solution was then prepared containing 82 grams per liter of this concentrate. To this were added 1.67 grams per liter of an aqueous nickel nitrate solution containing 12 percent nickel and 25.7 percent N Cold rolled steel panels were then cleaned with mineral spirits, dry rag wiped and rinsed for 15 seconds in hot water. The thus-cleaned panels were then immersed for 15 minutes in the working solution as described above, removed from the solution and rinsed in cold water for 30 seconds and then were blown dry with air. A 70 percent solution of nitric acid was then added to the working bath to raise the ratio of free P 0 to total P 0 and the procedure was repeated. This entire procedure was then repeated using a still higher ratio of free P 0 to total P 0 Throughout this process, the total acid in the working solution was maintained at about 5.4 and the free acid between'about 0.7 to 0.8. Using this procedure, the following results were obtained.

Average coating Amount of N H03 Ratio of free P20 thickness Solution added (grams/liter) to total P205 (millimierons) None 0. 3 2. 3 l. 17 0. 4 1. 5 2. 34 0. 5 1.3

EXAMPLE 15 The procedure of the preceding example was repeated with the exception that the manganese phosphate solution concentrate was prepared by combining 25.05 grams of the aqueous solution of nickel nitrate, 367.50 grams of the 50% solution of manganese nitrate, 205.5 grams of the manganese acid phosphate, 79.50 grams of the 75 percent phosphoric acid and 522.45 grams of water. This concentrate was then made into an aqueous working solution containing 80 grams per liter of the concentrate. The working solution was then divided into two portions, one of which had a ratio of free P 0 to total P 0 of 0.3 while the other had the ratio adjusted to 0.5 with nitric acid. Steel panels four inches by six inches were then vapor degreased with trichloroethylene and immersed in the working solutions for a period of 2.5 minutes at a temperature of 96 degrees centigrade, three panels being immersed in each of these solutions. The panels were then removed, rinsed with water and blown dry with air. The weight of coating deposited on each of these panels, in milligrams per square foot, was then determined. Using this procedure, the following results were obtained.

Coating weight obtained in milligrams/square foot Ratio of free P20 Solution to total P 05 Panel 1 Panel 2 Panel 3 A 0. 3 1, 394 1, 032 1, 008 B 0. 5 733 578 034 From the above examples, it is seen that where the ratio of free P 0 to total P 0 is not greater than 0.3, greatly increased coating weights and thicker coatings are obtained. Similarly, where the ratio is not less than 0.6, either no coating at all is formed or the coating obtained is undesirable because of its poor adhesion, and powdery characteristics. Additionally, the advantage to be obtained by including nickel ions in the manganese phosphate solution, in terms of the thinner coating obtained, is clearly shown in Examples 9-13.

Additionally, it is to be appreciated that by processing ferrous surfaces, e.g., articles having a surface of iron and/or steel, in accordance with the present invention, there is obtained a ferrous surface or a ferrous article having a manganese phosphate coating thereon of a thickness less than about 3 millimicrons. The production of ferrous surfaces and ferrous articles having such a thin, fine-grained and firmly adhering manganese phosphate coating has not been possible heretofore.

What is claimed is:

1. A composition suitable for use in producing a thin, finely grained firmly adhering coating on metallic surfaces which comprises an :acid manganese phosphate and an oxidizing agent, said composition having a ratio of free P 0 to total P 0 greater than 0.3 and less than 0.6.

2. The composition as claimed in claim 1 wherein the ratio of free P 0 to total P 0 is within the range of about 0.35 to about 0.55.

3. The composition as claimed in claim 2 wherein the ratio of free P 0 to total P 0 is within the range of about 0.40 to about 0.50.

4. The composition as claimed in claim 3 wherein the oxidizing agent is a nitrate.

5. A phosphatizing composition comprising an aqueous solution of an acid manganese phosphate and an oxidizing agent, said solution having a ratio of free P 0 to total P 0 greater than 0.3 and less than 0.6.

6. The composition as claimed in claim 5- wherein the oxidizing agent is a nitrate.

7. The composition as claimed in claim 6 wherein the aqueous solution contains from about 3 to about 20 gnams per liter manganese, from about 4 to about 25 grams per liter P 0 and from about 2 to about 50 grams per liter N0 8. The composition as claimed in claim 7 wherein there is also present nickel ions in an amount within the range of about 0.2 to about 1 gram per liter.

'9. The composition as claimed in claim 8 wherein the ratio of free P 0 to total P 0 is within the range of about 0.35 to 0.55.

10. The composition as claimed in claim 9 wherein the ratio of free P 0 to total P 0 is within the range of about 0.4 to about 0.5.

11. A method for producing a thin, fine-grained, firmly adhering manganese phosphate coating on a ferrous surface which comprises contacting the ferrous surface with an aqueous solution comprising an acid manganese phosphate and an oxidizing agent, which solution has a ratio of free P 0 to total P 0 greater than 0.3 and less than 0.6, and maintaining the ferrous surface to be coated in contact with the phosphate solution for a period of time sufiicient to effect the formation of the desired coating.

12. The process as claimed in claim 11 'wherein the oxidizing agent in the acid manganese phosphate solution is a nitrate.

13. The process as claimed in claim 12 wherein the acid manganese phosphate solution contains from about 3 to about 20 grams per liter manganese, from about 4 to about 25 grams per liter P 0 and from about 2 to about 50 grams per liter N0 14. The process as claimed in claim 13 wherein the manganese phosphate solution also contains nickel ions in an amount within the range of about 0.2 to about 1 gram per liter.

15. The process as claimed in claim 14 wherein the ratio of free P 0 to total P 0 is within the range of about 0.35 to about 0.55.

16. The process as claimed in claim '15 wherein the ratio of free P 0 to total P 0 is within the range of about 0.4 to about 0.5.

17. A ferrous article having a fine-grained, firmly adhering manganese phosphate coating thereon, which coating has a thickness of less than 3 millimicrons.

18. A ferrous article coated with a fine-grained, firmly adhering manganese phosphate coating, which coating has a thickness of less than 3 millimicrons.

References Cited UNITED STATES PATENTS 1,869,121 7/1932 Thompson et a1. 148-6.l5 2,516,139 7/1950 Mazia 148-6.l5 2,554,139 5/1951 Drysdale 148-6.16 2,636,257 4/1953 Ford 148-6.15 2,866,728 12/1958 .Warinner 148-6.15

ALFRED L. LEAVITT, Primary Examiner. J. H. NEWSOME, Assistant Examiner.

US. Cl. X.R. 252-49.7, 49.8 

